This proposal seeks to determine the effects of low level maternal inhalation exposure to benzene on fetal hematopoiesis using sensitive erythroid and granulocytic stem cell assays as well as the usual hematologic assays. Recent work in this laboratory has shown that exposure to 10 ppm benzene, the current TLV, affects committed granulocytic and erythroid stem cell proliferation. In the fetus hematopoietic stem cells are undergoing rapid cell division and the dividing blood cell pool is rapidly expanding during organogenesis. It is, therefore, likely that maternal exposure to even low levels of benzene would have a profound effect on fetal hematopoiesis. Groups of pregnant mice will be exposed to 100 ppm., 30 ppm. and 10 ppm. during days 6-16 of gestation. Blood cell parameters will be monitored on 16 day old fetuses, 2 day old neonates and 6 week old adults which are progeny of the exposed dams. Specifically the following questions will be studied after maternal exposure to low concentrations of benzene vapor: 1. What is the nature of the dose/response on fetal and neonatal hematopoietic stem cells? 2. Do any disruptions in hematopoiesis persist into young adulthood? 3. Are there any differences in the hematotoxic response between male and female fetuses? 4. Is the switch of functional hematopoiesis from the liver to the spleen and bone marrow affected in late fetal and neonatal life? 5. Is the normal progression of hemoglobin synthesis from fetal to adult type affected? The low level benzene exposure by inhalation should allow extrapolation of our findings to realistic human exposure conditions.